1. Field of the Invention
This invention pertains to charge transfer devices and more specifically to a range gate filter incorporating charge transfer devices, such as may be used in a moving target radar system. Operating principles of such systems are described in "Introduction to Radar Systems", M. I. Skolnik, published by McGraw-Hill Book Company, in particular Chapter 4.
2. Description of the Prior Art
Moving target radar uses the effect of doppler shift to isolate moving targets in the presence of zero doppler clutter as well as to determine the movement characteristics of such target, i.e., the speed and relative direction of movement. Long range radar may receive many returns from many targets. It is desirable that complete information be made substantially simultaneously for each of the targets.
A source sending a radar signal receives returns at different time intervals from a plurality of targets depending on the relative distance each target is from the source. That is, a close target return will return to the source relatively sooner than a far target return. It is apparent that the returns from a plurality of targets all resulting from one transmission are time sequential. By identifying each target with a time range, one target return may be discriminated from all other target returns.
In order to perform the association of each target with a particular time slot, the time of receipt of the target range of interest may be divided into time bins. Then each time bin may be separately interrogated to determine the characteristics of any targets that may be present.
The frequency of the signals within each time bin is indicative of relative movement of source and target, the time bin itself being indicative of location. The frequency carries this information since it is directly related by a well-known relationship to doppler shift and hence to relative movement.
Charge transfer devices have been employed in time demultiplexers to isolate one time bin or segment from the total target range. Essentially, such a demultiplexer receives all signal data in sequential order and progresses the data through a register. The register, loaded serially, is then outputted in parallel to a plurality of frequency filters to produce the separate information related to each time bin.
Separate conventional bandpass filters for each time bin site have been employed. It is not uncommon to have on the order of 60 time bins. To perform adequate filtering of each time bin in a conventional manner requires either a plurality of filters or an active filter that sweeps through the entire frequency range of interest. Such a filtering system is bulky and expensive. Time sharing filtering action to reduce the number of filters is not satisfactory since this inherently introduces delays in operation of the overall system and negatives the advantages of being able to detect multiple targets in a time shared return.
Therefore, it is a feature of this invention to provide a range gate filter comprising a time demultiplexer and a filtering system both employing charge transfer devices.
It is another feature of this invention to provide filtering of the frequencies in a moving target radar return using a transversal filter made from a charge transfer device.